I. The Complement System
The complement system is a complex interaction of at least 25 plasma proteins and membrane cofactors which act in a multistep, multiprotein cascade sequence in conjunction with other immunological systems of the body to defend against intrusion of foreign cells and viruses. Complement proteins represent up to about 10% of globulins in normal serum of humans and other vertebrates. Complement components achieve their immune defensive functions by interacting in a series of intricate but precise enzymatic cleavage and membrane binding events. The resulting complement cascade leads to the production of products with opsonic, immunoregulatory, and lytic functions.
There are two main routes of complement activation: the classical pathway and the alternative pathway. These pathways share many components, and while they differ in their initial steps, they converge and share the same "terminal complement" components responsible for the activation, attack, and/or destruction of target cells.
The classical complement pathway is typically initiated by antibody recognition of and binding to an antigenic site on a target cell. The alternative pathway is usually antibody independent, and can be initiated by certain molecules on pathogen surfaces. Both pathways converge at the point where complement component C3 is cleaved by an active protease (which is different in each pathway) to yield C3a and C3b. The active protease, which is referred to as C3 convertase, comprises complement components C2bC4b for the classical pathway and complement components C3bBb for the alternative pathway.
C3a is an anaphylotoxin that can induce degranulation of mast cells, resulting in the release of histamine and other mediators of inflammation. C3b has multiple functions. As opsonin, it binds to bacteria, viruses and other cells and particles and tags them for removal from the circulation. C3b can also form a complex with other components unique to each pathway to form classical or alternative C5 convertase, which cleaves C5 into C5a (another anaphylatoxin), and C5b.
C5a, like C3a, is a potent anaphylatoxin which can cause the activation of granulocytes and platelets. Additionally, C5a is a chemoattractant for neutrophils and also mediates mast cell histamine release and resulting smooth muscle contraction. C5b, on the other hand, combines with C6, C7, and C8 to form the C5b-8 complex at the surface of the target cell. Upon binding of C9 the membrane attack complex (MAC, C5b-9) is formed. When sufficient numbers of MACs insert into target cell membranes, the openings they create mediate rapid lysis of the target cells. Lower, non-lytic concentrations of MACs can produce other effects. In particular, membrane insertion of small numbers of the C5b-9 complexes into endothelial cells and platelets can cause potentially deleterious cell activation. In some cases activation may precede cell lysis.
Control of the complement system is necessary in order to prevent destruction of autologous cells. Since 1900 it has been known that complement-mediated cytolysis is not efficient when the complement and the target cells are from the same species. (Bordet, 1900.) Studies on the susceptibility of non-human cells to complement-mediated lysis have shown that such cells are readily lysed by human complement while they are generally resistant to lysis by complement derived from the same species. (Houle et al., 1988). This phenomenon is referred to in the art as "homologous species restriction of complement-mediated lysis." The mechanism by which such restriction takes place has been at least partially revealed by a series of experiments in which complement regulatory proteins have been identified that serve to protect cells from homologous complement-mediated damage. (Rollins et al., 1991).
II. C3 Inhibitor Proteins
A family of cell-surface proteins with shared structural features has been described each of whose actions impact on C3b.
Decay accelerating factor (DAF or CD55) exists on all cells, including red blood cells. DAF is a single chain, 70 kDa glycoprotein that is linked to the cell membrane via a glycophosphatidyinositol (GPI) moiety which inserts into the outer leaflet of the plasma membrane bilayer.
DAF regulates complement activation at the C3 convertase stage by preventing the assembly of the C3 convertases of both the classical and alternative pathways (Medof et al., 1984; Fujita et al., 1987). Thus, DAF prevents the formation of the anaphylactic cleavage fragments C3a and C5a, in addition to inhibiting amplification of the complement cascade on host cell membranes.
DAF has been shown to act exclusively in an intrinsic manner on cells, protecting only the cell on whose surface it resides while having no effect on neighboring cells. After extraction from human red blood cells, DAF reincorporates into cell membranes and is biologically active. Both membrane and secreted forms of DAF have been identified and their cDNAs have been cloned and characterized (Moran et al., 1992).
The nucleotide and amino acid sequences for human DAF are set forth in the Sequence Listings as SEQ ID NO:1.
Membrane cofactor protein (MCP or CD46) exists on all cells except red blood cells. MCP is a type I transmembrane glycoprotein that binds to C3b. MCP acts as a cofactor in the factor I-mediated cleavage of C3b and C4b deposited on self tissue. Therefore, the presence of bound MCP activates molecules that cleave C3b into inactive fragments, preventing the potentially cytolytic accumulation of C3b. Nucleotide and amino acid sequences for MCP can be found in Lublin, et al., 1988.
Complement receptor 1 (CR1 or CD35) is found on erythrocytes as well as a select group of leukocytes, including lymphocytes, neutrophils, and eosinophils. CR1 is a 190-280 kDa transmembrane protein that triggers the proteolytic degradation of membrane bound C3b molecules with which it comes in contact. It also promotes the clearance of immune complexes. Nucleotide and amino acid sequences for CR1 can be found in Wong, et al., 1985.
Factor H and C4b-binding protein each inhibit the activity of alternative pathway C3 convertase. Nucleotide and amino acid sequences for factor H can be found in Ripoche, et al., 1988; nucleotide and amino acid sequences for C4b-binding protein can be found in Chung, et al., 1985.
The genes encoding all of these C3 inhibitory proteins have been mapped to the long arm of chromosome 1, band 1q32, and constitute a locus designated the RCA (Regulators of Complement Activity) gene cluster. Notable in the molecular structure of these C3 inhibitory proteins is a common structural motif of approximately 60 amino acids designated the SCR (short consensus repeat), which is normally present in multiple copies that are not necessarily identical. See Perkins et al. 1988; Coyne, et al., 1992.
The SCR motif of these C3 inhibitory proteins has four conserved cysteine residues and conserved tryptophan, glycine, and phenylalanine/tyrosine residues. The SCRs are usually followed by a long serine/threonine rich region.
In DAF and MCP, the SCRs are known to encode functional domains necessary for full complement inhibitory activity (Adams, et al., 1991). DAF is composed of 4 SCRs juxtaposed to a serine/threonine rich region on the carboxyl terminal side of the SCRs. Most, if not all, of the functional domains are reported to reside in SCRs 2 through 4 (Coyne et al., 1992). In SEQ ID NO:1, the 4 SCRs of DAF comprise amino acid 1 through amino acid 61 (SCR 1), amino acid 62 through amino acid 125 (SCR 2), amino acid 126 through amino acid 187 (SCR 3), and amino acid 188 through amino acid 250 (SCR 4), Lublin, et al., 1989.
The phrase "C3 inhibitory activity" is used herein to describe the effects of C3 inhibitor molecules of the foregoing types on the complement system and thus includes activities that lead to disruption of the C3 convertase complex and/or activities that are responsible for the degradation of C3b.
III. C5b-9 Inhibitor Proteins
The archetypical C5b-9 inhibitor protein is the human glycoprotein known as CD59. The nucleotide and amino acid sequences for human CD59 are set forth in the Sequence Listings as SEQ ID NO:2.
CD59 is found associated with the membranes of cells including human erythrocytes, lymphocytes, and vascular endothelial cells. It serves to prevent assembly of functional MACs and thus protects cells from complement-mediated activation and/or lysis. CD59 has an apparent molecular mass of 18-21 kilodaltons (kD) and, like DAF, is tethered to the outside of the cell membrane by a GPI anchor. See, for example, Sims et al., U.S. Pat. No. 5,135,916.
CD59 appears to function by competing with C9 for binding to C8 in the C5b-8 complex, thereby decreasing the formation of the C5b-9 membrane attack complex. (Rollins et al., 1990.) CD59 thus acts to reduce both cell activation and cell lysis by terminal complement MACs. This activity of CD59 is for the most part species-restricted, most efficiently blocking the formation of MACs under conditions where C8 and C9 are derived from homologous (i.e., human) serum. (Venneker et al., 1992.)
The assimilation of purified CD59 into the plasma membrane of non-human erythrocytes (which appear to be protected from homologous complement lysis by the action of their own cell surface complement inhibitor proteins) and oligodendrocytes (brain cells which are believed to be protected less, if at all, by cell surface proteins, but may be protected in vivo by the blood brain barrier) has shown that CD59 can protect these cells from lysis mediated by human complement. (Rollins, et al., 1990; Rollins, et al., 1991; Stefanova, et al., 1989; Meri, et al., 1990; Whitlow, et al., 1990; Okada, et al., 1989; and Wing, et al., 1992).
cDNAs encoding CD59 have been cloned and the structure of the CD59 gene has been characterized (Davies, et al., 1989; Okada, et al., 1989; Philbrick, et al., 1990; Sawada, et al., 1990; and Tone, et al., 1992). Non-human mammalian cells transfected with the cloned CD59 cDNA, and thereby expressing the human CD59 protein on their cell surfaces, have been shown to gain resistance to complement-mediated cell lysis (Zhao, et al., 1991; and Walsh, et al., 1991).
CD59 has been reported to be structurally related to the murine Ly-6 antigens (Philbrick, et al., 1990; and Petranka, et al., 1992). The genes encoding these antigens, also known as T-cell activating proteins, are members of the Ly-6 multigene family, and include Ly-6A.2, Ly-6B.2, Ly-6C.1, Ly-6C.2, and Ly-6E.1. The gene encoding the murine thymocyte B cell antigen ThB is also a member of this family (Shevach, et al. 1989; and Gumley, et al., 1992).
A distinguishing feature of the amino acid sequences of the proteins of the Ly-6 family is the arrangement of their cysteine residues. Cysteine residues of many proteins form a structural element referred to in the art as a "cysteine backbone." In those proteins in which they occur, cysteine backbones play essential roles in determining the three dimensional folding, tertiary structure, and ultimate function of the protein molecule.
The proteins of the Ly-6 multigene family, as well as several other proteins share a particular cysteine backbone structure referred to herein as the "Ly-6 motif". For example, the human urokinase plasminogen activator receptor (uPAR; Roldan, et al., 1990) and one of several squid glycoproteins of unknown function (Sgp2; Williams, et al., 1988) contain the Ly-6 motif.
Subsets of proteins having the Ly-6 motif can be identified by the presence of conserved amino acid residues immediately adjacent to the cysteine residues. Such conservation of specific amino acids within a subset of proteins can be associated with specific aspects of the folding, tertiary structure, and ultimate function of the proteins. These conserved patterns are most readily perceived by aligning the sequences of the proteins so that the cysteine residues are in register.
As discussed fully in copending, commonly assigned, U.S. patent application Ser. No. 08/105,735, filed Aug. 11, 1993, by William L. Fodor, Scott Rollins, Russell Rother, and Stephen P. Squinto, and entitled "Complement Inhibitor Proteins of Non-human Primates", the relevant portions of which are incorporated herein by reference, and in Rother, et al., 1994, a series of non-human primate C5b-9 inhibitory proteins have been identified which are characterized by a cysteine backbone structure which defines a specific subset of the general Ly-6 motif.
Specifically, these non-human primate CIPs include polypeptides comprising a cysteine backbone with a Ly-6 motif characterized by the formula: EQU Cys--X.sub.2 --Cys--X.sub.6-9 --Cys--X.sub.5 --Cys--X.sub.6 --Cys--X.sub.12 --Cys--X.sub.5 --Cys--X.sub.17 --Cys--X.sub.0 --Cys--X.sub.4 --Cys.(1)
In addition, the non-human primate C5b-9 inhibitory proteins include amino acid sequences conforming to the following formula: EQU Cys--X.sub.2 --Cys--Pro--X.sub.5-8 --Cys--X.sub.4 --Asn--Cys--X.sub.5 --(Thr or Ser)--Cys--X.sub.11 --(Gln or Arg)--Cys--X.sub.4 --(Asn or Asp)--Cys--X.sub.17 --Cys--X.sub.0 --Cys--X.sub.4 --Cys. (2)
In both formulas, the X in X.sub.n indicates a peptide containing any combination of amino acids, the n in X.sub.n represents the length in amino acid residues of the peptide, and each X at any position can be the same as or different from any other X of the same length in any other position.
As discussed fully in commonly assigned, copending PCT application Ser. No. PCT/US 93/00672, filed Jan. 12, 1993, by Bernhard Fleckenstein and Jens-Christian Albrecht, and entitled "Complement Regulatory Proteins of Herpesvirus Saimiri", the relevant portions of which are incorporated herein by reference, and in Albrecht, et al., 1992, a protein of the herpesvirus saimiri having C5b-9 inhibitory activity has been discovered (referred to herein as "HVS-15"). This viral protein has the Ly-6 motif which is characteristic of the non-human primate C5b-9 inhibitory proteins discussed above, i.e., its structure is described by formulas (1) and (2) above.
The phrase "C5b-9 inhibitory activity" is used herein to describe the effects of C5b-9 inhibitor molecules of the foregoing types on the complement system and thus includes activities that lead to inhibition of the cell activating and/or lytic function of the membrane attack complex (MAC).
V. Complement Associated Pathologies
Human studies and studies using animal models of human disorders have implicated CIPs in the pathologies associated with a number of disorders, including the following.
Transplantation: Intrinsic activation of complement attack via the alternative pathway during storage of donor organs is responsible for certain problems associated with organ transplantation which arise as a result of endothelial cell stimulation and/or lysis by the C5b-9 MAC (Brasile, et al. 1985). Ex vivo complement attack leads to reduced vascular viability and reduced vascular integrity when stored organs are transplanted, increasing the likelihood of transplant rejection.
Ten percent of allogeneic transplanted kidneys with HLA-identical matches are rejected by in vivo immunologic mechanisms (Brasile, et al. 1987). In 78% of the patients who reject organs under these conditions, cytotoxic antibodies binding to molecules on the surfaces of vascular endothelial cells are seen (Brasile, et al., 1987). Such antibody cytotoxicity is mediated by complement attack, and is responsible for the rejection of transplanted solid organs including kidneys and hearts (Brasile, et al., 1987; Brasile et al., 1985). Antibody primed, complement-mediated rejection is usually rapid and irreversible, a phenomenon referred to as hyperacute rejection.
In the xenogeneic setting, as when non-human organs are transplanted into human patients, activation of complement attack by antibodies directed against molecules on the surfaces of endothelial cells lining the vessels of the donor organ is almost always observed. The prevalence of such xenoreactive antibodies accounts for the nearly universal occurrence of hyperacute rejection of xenografts (Dalmasso, et al., 1992). Old world primates, including humans, have high levels of preexisting circulating "natural" antibodies that predominantly recognize carbohydrate determinants expressed on the surface of xenogeneic cells from discordant species. Recent evidence indicates that most of these antibodies react with galactose in an .alpha.1-3 linkage with galactose.(Gal(.alpha.1-3)Gal) (Sandrin, et al., 1993).
Old world primates lack the appropriate functional .alpha.-1,3-galactose transferase and thus do not express this carbohydrate epitope. Therefore, following transplantation of a vascularized xenogeneic donor organ, these high-titer antibodies bind to the Gal(.alpha.1-3)Gal epitope on the vascular endothelium and activate the recipient's complement through the classical pathway. The massive inflammatory response that ensues from activation of the complement cascade leads to the destruction of the donor organ within minutes to hours.
Xenoreactive antibodies are not exclusively responsible for hyperacute rejection of discordant organs in all cases. For example, erythrocytes from some species can activate human complement via the alternative pathway and newborn piglets raised to be free of preformed antibodies reject xenografts almost immediately. It is therefore likely that in some species combinations, activation of the alternative complement pathway contributes to graft rejection.
Endogenously-expressed, membrane-associated complement inhibitory proteins normally protect endothelial cells from autologous complement. However, the species restriction of complement inhibitors makes them relatively ineffective with respect to regulating discordant xenogeneic serum complement. The lack of effective therapies aimed at eliminating this antibody and complement-mediated hyperacute rejection presents a major barrier to the successful transplantation of discordant animal organs into human recipients.
Recently, a report on a baboon-to-human liver transplant has been published in which the xenogeneic donor organ failed to exhibit signs of hyperacute rejection (Starzl, et al., 1993). The low levels of anti-baboon antibodies likely to be present in human blood make hyperacute responses less likely. However, it is believed that recently discovered baboon CIPs, which have been shown to be related to CD59 and to be effective against human complement, also played a role in maintaining the integrity of this xenotransplanted organ. (See U.S. patent application Ser. No. 08/105,735, referred to above.)
The lack of hyperacute rejection seen in the baboon to human xenotransplant discussed above suggests that complement inhibitor proteins effective against human complement may, in combination with other anti-rejection strategies, allow safe and effective xenotransplantation of transgenic animal organs expressing such proteins into human patients.
Paroxysmal Nocturnal Hemoglobinuria: A complement mediated disease that involves the alternative pathway of complement activation is the stem cell disorder paroxysmal nocturnal hemoglobinuria. Complement inhibitory proteins, including CD59, are absent from the membranes of the most hemolytically sensitive erythrocytes found in patients with this disease. The lack of these proteins is thought to potentiate the complement-mediated lysis of red blood cells that characterizes the disease (see Venneker et al., 1992). The use of chimeric terminal complement inhibitor proteins in the treatment of PNH cells is discussed in copending, commonly assigned, U.S. patent application Ser. No. 08/206,189, entitled "Method for the Treatment of Paroxysmal Nocturnal Hemoglobinuria," which is being filed concurrently herewith in the names of Russell Rother, Scott Rollins, Seth A. Fidel, and Stephen P. Squinto.
VI. CIPs with Modified Membrane Anchors
Work has been performed in which CIPs with modified membrane anchors have been generated in order to study the functional consequences of altering the means of attachment of GPI-anchored proteins to the outer cell surface. In these studies, the native cell surface anchoring of the CIPs has been varied from their natural GPI anchors by substitution of other anchoring moieties (Su, et al., 1991; and Lublin, et al., 1991).
For example, derivatives of DAF, containing amino acids 1-304 of DAF fused to the transmembrane domain of MCP (i.e., amino acids 270-350 of MCP) or to the transmembrane domain of the human major histocompatibility protein HLA-B44 (i.e., amino acids 262-338 of HLA-B44) have been reported to retain levels of function equivalent to native DAF (Lublin, et al., 1991).
Derivatives of CD59, containing amino acids 1-77 of CD59 fused to the transmembrane domain of MCP (i.e., amino acids 270-350 of MCP) have been shown to retain levels of function equivalent to native CD59 in copending, commonly assigned, U.S. patent application Ser. No. 08/205,720, entitled "Terminal Complement Inhibitor Fusion Genes and Proteins," which is being filed concurrently herewith in the names of Russell Rother, Scott Rollins, and Stephen P. Squinto.